The present invention relates to a method of obtaining an image and, more particularly, a method of obtaining a substantially linear representation of the brightness of an image having a wide dynamic range.
Digital electronic cameras and other similar imaging devices and systems often use CCD (charge coupled device) sensors, and work by converting incident photons into charge and accumulating the charge in each pixel for the duration of the exposure. Like all other imaging devices, including conventional wet photography and other digital systems, such imaging devices impose limits on the dynamic range of the signal they can capture. In particular, image details in the regions which are either too dark or too bright cannot be captured. In such digital devices, the charge in each cell on to which part of an image is imposed, is read out and converted into a number representing its intensity or brightness via an analogue to digital converter. Therefore, if the image is too bright, or the exposure time is too long, then saturation occurs and the brightness of the image is clipped to the maximum representable intensity and no detailed information is available in these areas. This imposes an upper limit on the detected brightness. Conversely, if the image is too dark any signal is indistinguishable above the quantisation noise in the analogue to digital converter and therefore detail is lost in these dark areas.
An important characteristic of this clipping process is that it is destructive and destroys information which cannot subsequently be recovered from the recorded image. Therefore any attempt to capture the information that would otherwise be missing, must be made at the time of capture of the image. The resulting image requires a wider dynamic range than that of the basic sensor and analogue to digital converter. It is possible, then, to correct overall over or under exposure by adjusting the time over which the sensor is accumulating charge so that the full dynamic range of the digital imaging system is utilised. However, whilst this uses the available dynamic range to best effect, it cannot assist where there are both very bright and very dark regions, for example, in an electrophoresis gel, in the same field of view as both lengthening and reducing the exposure time cannot be carried out simultaneously. In these circumstances it is possible to capture either the bright areas or the dark areas accurately, but not both simultaneously.
Now, it is evident that, under such circumstances, a series of images captured using different exposure times will contain all of the available information. Indeed a technique well known in the art involves simply averaging together a series of images recorded with steadily increasing exposure times. This can be shown to result in an image which is approximately proportional to the logarithm of the intensity. Such a result may be pleasing to the eye in that very bright regions do not suppress the detail of very dark regions, but such images lack the crucial linear quality often required for quantitative analysis.
It is possible to obtain camera systems having an inherent high dynamic range and whose output can be digitised to a 16 bit resolution. Such systems can achieve the dynamic range necessary to image both very dark and very light areas without saturation, but they are both very expensive and generally slow to operate with regard to focussing and adjusting the field of view. The advantages of the present invention are that an inexpensive sensor and digitizer may be used, and the readout can be rapid, facilitating convenient, fluid adjustment of focus.
The ideal digital output ixy from an analogue to digital converter of a true image of intensity ixy is given byv′xy=KTixy+C where T is the exposure time, K is the overall gain of the system, and C is an offset. However, due to the saturation at the high and low ends of the range the true output ixy is constrained by:
      v    xy    =      {                                                    ⁢                                          KTi                xy                            +              C                                                                          v              min                        <                          v              xy              ′                        <                          v              max                                                                                      v              max                        ⁢                                                  ⁢            when                                                            ⁢                                          v                xy                ′                            ≥                              v                max                                                                                                  ⁢                          v              min                                                                        ⁢                                          v                xy                ′                            ≤                              v                min                                                        In order to reduce the effects of noise an average over a series of images with different exposure times Tn can be taken, giving a superior estimate of the true image ixy:
            i      ^        xy    =            1      n        ⁢                  ∑        n            ⁢                          ⁢              (                                            v                              n                ,                xy                                      -            C                                KT            n                          )            
Averaging of this kind is well known in the art but it does not resolve the problem of saturated areas and dark areas and therefore the present invention discards the values for these pixels from the average, such that only pixels having significant values, that is those well away from the dark and light limits, are considered when averaging the images together.
The present invention is aimed at overcoming the shortcomings of the prior art methods.